Difference between related url and alternative url

[Tomek]

Hi,

I'm developing a custom ecommerce store, and I would like to create 
friendly URLs for products and its categories. I have noticed that I can 
connect some product or categories with a content like "Alternative URL" 
or "Related URL" - thanks for it I can typed friendly URL to the system.

But I don't know what it is the difference between "Alternative URL" and 
"Related URL" types of content? Can somebody explain me when I should 
use type "Alternative URL" and when I should use type "Related URL"?

Tomek